The invention relates to a method for controlling (open loop and/or closed loop) the pressure in an underpressure store of a pneumatically operated servo system wherein the underpressure store is charged with underpressure via an intake manifold of an internal combustion engine.
A method of the above-mentioned type is used, for example, in motor vehicles known in the marketplace. The brake boosters of such motor vehicles operate pneumatically and obtain their ancillary energy from an underpressure store. The underpressure store is connected via a tap line to the intake manifold of the engine of the motor vehicle. An underpressure is present in the intake manifold of the motor vehicle during operation of the engine. For this reason, the underpressure store an be evacuated and the underpressure can be made available which is required for the operation of the brake booster.
However, it is problematic that the underpressure in the intake manifold of modern engines is relatively low at least in the emission optimized or consumption optimized operating state. This is so primarily in engines having fuel direct injection wherein the throttle flap in the intake manifold is essentially open in the optimal operating state and therefore the underpressure in the intake manifold is relatively low.
Furthermore, in engines having E-gas (electronic accelerator pedal), the ignition efficiency in specific operating states (for example, during the warm-up phase) is deteriorated for the purpose of heating the catalytic converter and the throttle flap is opened further. A deteriorated ignition efficiency increases the temperature of the exhaust gas. The also resulting reduction of the torque is compensated by an additional charge in such engines, for example, after a braking action, when the underpressure in the underpressure store becomes less because of an air quantity flowing from the brake booster into the underpressure store, a measure is initiated which reduces the pressure in the intake manifold to the extent that the underpressure store can be evacuated in an adequate manner. This measure comprises, in general, that the throttle flap is closed and the engine is therefore throttled.
Such a measure means (for example, in direct injection gasoline engines) that the engine is operated in the so-called homogeneous mode of operation and not in the so-called stratified mode of operation whereby the consumption is increased. Also, the emission performance is deteriorated especially during the warm-up phase of the engine of such a motor vehicle because the heating of the catalytic converter with the throttle lap closed takes place slower because of the better ignition efficiency and therefore the catalytic converter for a longer time does not operate at its optimal operating temperature.
Alternatively or additionally, an electrical suction pump can be provided with which the underpressure store can be evacuated. Such a suction pump has, however, a limited total operating time. For this reason, the pump should be switched on as infrequently as possible.
In view of the above, it is an object of the invention to provide a method of the kind initially referred to herein which is so improved that the consumption and also the exhaust-gas performance of a corresponding internal combustion engine are improved. It is a further object of the invention to provide a method wherein a built-in suction pump, if present, is not subjected to excessive wear.
The method of the invention is for controlling the pressure in an underpressure store of a pneumatically operated servo system of a vehicle having an internal combustion engine. The method includes the steps of: applying an underpressure pSR to the underpressure store via a suction line; setting a higher desired pressure (p1) for the underpressure store in a first state of the servo system wherein the servo system is not actuated and wherein no activation is expected; and, setting a lower desired pressure (p2) for the underpressure store in a second state wherein the servo system is actuated or an actuation thereof is expected.
The above feature according to the invention is based on the realization that an underpressure is often present in the underpressure store which is not even necessary for a reliable operation of the servo system in many operating situations of the motor vehicle. With respect to the example of a brake booster, it has been determined that it is fully sufficient that a certain underpressure is present in the underpressure store at the start of the braking operation and therefore a certain support of the braking operation is possible via the servo system. This underpressure must be present in the quiescent state of the servo system in which this system is not actuated and no actuation is expected. This underpressure can be considerably less than that underpressure which is required in order to ensure a permanent and intense actuation of the servo system.
A lower desired pressure in the underpressure store means, however, automatically that the interventions with which an underpressure is established in the intake manifold of the engine can take place less frequently and/or in a less disadvantageous manner for the consumption or emission performance. This underpressure is required for the evacuation of the underpressure store. Also, an electrical suction pump which is possibly present must be less frequently switched on.
The use of the method of the invention is especially advantageous in the operation of an underpressure store at higher elevations. Since there the ambient pressure is already relatively low, under certain circumstances, measures must be taken continuously in order to further reduce the pressure in the intake manifold of the engine and thereby make available the pressure difference which is necessary in order to be able to charge the underpressure store with underpressure via the intake manifold. In such a case, the fuel consumption of the engine would be continuously increased or the emission performance would continuously deteriorate without the use of the method of the invention. Alternatively, a possibly present electrical section pump would have to be switched on continuously under certain circumstances which would clearly reduce the service life thereof. This is prevented with the method of the invention.
In order to have an adequate underpressure available at that moment at which the servo system is intended to be activated, it is advantageous when the pressure in the underpressure store corresponds to the lower desired pressure already shortly therebefore. A simple possibility for predicting the actuation of the servo system with high probability is afforded by the position of the accelerator pedal. This is accounted for in a further embodiment wherein an actuation of the servo system is expected when the work position of the accelerator pedal drops below a specific minimum value.
Especially advantageous is also when the desired pressures are determined in dependence upon the ambient pressure. In this way, for example, for a low ambient pressure, the required pressure difference (underpressure) between the underpressure store and the outside atmosphere is reduced which likewise leads to the situation that interventions into the flow in the intake manifold with which the required underpressure should be made available for evacuating the underpressure store or driving an electric pump can take place less frequently or with less intensity.
In another embodiment of the invention, a measure for reducing the pressure is initiated when the pressure in the underpressure store exceeds a limit value and then, when the pressure in the underpressure store drops below a limit value, a corresponding measure, which reduces the pressure, is ended. One of the two limit values corresponds to the desired value and the desired value distinguishes itself from the other limit value by a hysteresis factor. In this way, a tolerance range for the desired pressure in the underpressure store is provided with which the condition is prevented that measures for dropping the underpressure are initiated already for a slight pressure increase in the underpressure store.
Furthermore, various advantageous measures, which are easy to realize, are mentioned with which the underpresaure in the underpressure store can be reduced. The following are appropriate, namely: closing of a throttle flap in the intake manifold and/or switching on a suction pump.
The invention further relates to a computer program which is suitable for carrying out the above-mentioned method when it is executed on a computer. Here, it is especially advantageous if it is stored on a memory, especially on a flash memory.
The invention relates finally to an arrangement for controlling the pressure in an underpressure store of a pneumatically operated servo system of a vehicle having an internal combustion engine. The underpressure store is connected to a suction line. According to the invention, it is provided that the arrangement is so configured that it presets a higher desired pressure p2 in a rest position of the servo system wherein the system is not actuated and no actuation thereof is expected and presets a lower desired pressure p1 for the underpressure store 24 in a condition in which the servo system is actuated or an actuation thereof is expected.